Combination advanced corneal topography/wave front aberration measurement

ABSTRACT

A method for the simultaneous measurement of the anterior and posterior corneal surfaces, corneal thickness, and optical aberrations of the eye. The method employs direct measurements and ray tracing to provide a wide range of measurements for use by the ophthalmic community.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to apparatus for use in determining thefront and back contours of the cornea of a human eye and thusfacilitating the diagnosis and evaluation of corneal anomalies, designand fitting of contact lens, and the performance of surgical procedures.The present invention also relates to the field of measurement of therefractive characteristics of an optical system, and more particularly,to automatic measurement of the refractive characteristics of the humanor other animal eye and to corrections to the vision thereof.

2. Background of Related Art

Corneal Front Surface Measurements

The cornea, being the front surface of the eye, provides the majority ofthe refracting power (about ⅔) of the eye and is important to quality ofvision. Recently, a number of corneal surgical techniques have beendeveloped for correcting visual deficiencies, such as near-sightedness,far-sightedness and astigmatism. In order to assist with such surgicaltechniques, a number of devices have been proposed or developed toevaluate the topography, i.e., the shape or curvature, of the cornea. Inaddition, such corneal topography techniques are useful for fittingcontact lenses and for the diagnosis and management of cornealpathologic conditions, such as keratoconus and other ectasias. Forexample, prior to performing a corneal surgical technique to correct arefractive error, the patient is preferably screened using a cornealtopography device to rule out the possibility of subclinicalkeratoconus.

Corneal topography is typically measured using a series of concentriclighted rings, known as a keratoscope pattern, shown in FIG. 12. In atypical embodiment, the keratoscope pattern (reflected image of rings onCCD) is created by a keratoscope target, consisting of illuminatedconcentric rings which emit light rays which are projected onto thecornea of the patient's eye. Light rays are reflected off the patient'scornea, and a portion of the light is captured by a camera lens andfocused onto a CCD. A computer is utilized to analyze the captured imageto identify any distortions in the captured image and thus calculate anydeformations in the patient's cornea.

While conventional corneal topography devices have achieved significantsuccess, such devices suffer from a number of limitations, which, ifovercome, could significantly enhance their accuracy and utility. Forexample, commercially available topography devices, such as the designillustrated in FIG. 12, typically measure the topography of only aportion of the cornea. In the design shown in FIG. 12, the light beam isemitted from a large, backlit keratoscope target and is then reflectedoff the cornea. Thereafter, a portion of the light reflects off thecornea and is focused by the camera lens at the center of thekeratoscope target onto the CCD. Using this same technique to attempt toimage the peripheral portion of the cornea would require a very largeextended keratoscope target as shown in the “imaginary extension” ofFIG. 12. This imaginary extension could not be realized in a real systemdue to size and interference with the subject's head. Therefore, suchprior art devices are unable to measure the peripheral cornea.

To overcome this problem, other corneal topography devices haveattempted to capture the light rays reflected from the peripheralportions of the cornea by designing a very small keratoscope target inthe shape of a cylinder or cone, as shown in FIG. 13, encompassing theperipheral cornea. In this manner, light rays emitted by the cylindricalor conical keratoscope target will form a pattern of illuminated ringswhich will be reflected off the cornea. The reflected light rays,including light rays reflected off the peripheral portions of thecornea, will be captured by the lens and imaged onto the CCD. For thisstrategy, however, the cylindrical or conical keratoscope target must bepositioned very close to the eye, and thereby tends to impinge on thepatient's brow and nose. In addition to being potentially uncomfortableand potentially contributing to the spread of disease, the closeapproach of the keratoscope target makes the design very error-prone, asa slight error in alignment or focusing causes a large percentage changein the position of the keratoscope rings relative to the eye and, hence,a large error in the measurement of the cornea.

In addition, current systems tend to provide poor pupil detection and donot accurately measure non-rotationally symmetric corneas, such as thosewith astigmatism. The location of the pupil is particularly important inplanning surgical procedures for correcting visual deficiencies. Incurrent systems, pupils are typically detected by deciphering the borderof the pupil from the image of the keratoscope rings. This isparticularly difficult with conventional designs, however, as theintensity transition from the black pupil to a dark iris is minimalcompared to the intensity transition from a bright keratoscope ringimage to a dark interring spacing. As a result, the pupil detectionalgorithms in current systems often fail or provide poor results.

In a recent corneal topography advancement, Malone (U.S. Pat. No.5,873,832) describes a technique which utilizes a virtual image of akeratoscope pattern. The topography system reflects a structured lightpattern off the cornea where light rays travel perpendicular to thecornea. In this manner, more of the peripheral cornea is imaged. Thegeometry of these reflected rays is similar to that of the innermostrays of the traditional corneal topography system. It is well known thatthe innermost data of traditional corneal topography systems haverelatively low accuracy, so it is likely that this new technique willhave lower accuracy than that currently provided by the commerciallyavailable corneal topography systems.

In the present invention, we overcome these problems of corneameasurement coverage and accuracy using a novel skew-view cornealanalysis technique as explained below. We make use of three cameras aswas detailed by Sarver (U.S. Pat. No. 5,847,804). While Sarverspecifically used a front-view camera and two orthogonal side viewcameras (only one of which was used during an exam), the presentinvention uses a front-view camera and a left- and right-camera orientedat 45 degrees to the optical axis of the front-view camera and all threecameras are used for each exam.

Cornea Back Surface Measurements and Cornea Thickness

Corneal thickness is commonly measured using an ultrasound technique.The hand held A-scan ultrasound probe produces a single-pointmeasurement of the thickness of the cornea. This single point is, inreality, the average thickness of an area of several square millimetersin extent. Because the location of the measurement is dependent upon theoperator's positioning, the location of the measurement is not exactlyrepeatable, hence the data is variable as well.

Another method is the scanning slit technique reported in Snook (U.S.Pat. No. 5,512,966), Knopp (U.S. Pat. No. 5,870,167), and Lempert (U.S.Pat. No. 5,404,884). In these techniques a slit of light is passedthrough the cornea and the interface of the slit with the front and backsurfaces is evaluated from a digitized image. Using this information andan estimate of the index of refraction of the cornea, the thickness ofthe cornea can be estimated. By scanning the slit over several portionsof the cornea, the thickness of a significant portion of the cornea canbe obtained. Since the diffuse interaction of the light slit and thecornea can be ill-defined, the image processing will not be exact and sothe measurements will contain some amount of error. These techniquesalso suffer from the characteristic that a large number of images mustbe obtained and processed to estimate a large portion of the cornea. Theresult is a large amount of data to process and store as well as thecomplexity of registration of the images due to movement of the eyeduring the acquisition period. Even though advanced data compressiontechniques exist such as that developed by Sarver (U.S. Pat. No.5,418,714), the images still must be decompressed prior to processing.

In the present invention we take a completely novel approach whicheliminates these shortcomings. Using the same three cameras as used inthe cornea front surface calculation, we image the pupil contour. Alight pattern in the shape of a cross, similar to two simultaneouslyprojected orthogonal slits, is projected onto the cornea. As illustratedin FIG. 14, this pattern is viewed by the front-view camera (C3) toimage the horizontal portion of the cross and the left- and right-viewcameras (C1 and C2) to image the vertical portion of the cross. Thisprovides a starting point for the corneal thickness and corneal backsurface measurements. Then, knowing the front surface, and the startingpoint provided by the horizontal and vertical thickness data, we findthe back surface such that corresponding rays from the three cameraviews would trace through the cornea and intersect at the pupil contour.Details of this new process are presented below.

Wave Front Aberration Measurement of the Eye

In contrast with man-made optical systems, human and animal eyes areoptical systems in which the individual internal components of a giveneye are not normally separately accessible for either direct measurementor adjustment, the output of the optical system is not directlyaccessible for analysis and the characteristics of individual componentschange over time with growth, aging and other factors.

The most common reason for measuring the optical characteristics of ahuman eye is to determine a prescription for corrective lenses tocorrect vision problems. Such measurements of the opticalcharacteristics of the eye have long been made by the actual or apparentsubstitution of lenses with various correction factors with the patientindicating the effect of each substitution in terms of the image beingclearer or fuzzier. This technique determines an overall correction forthe optical characteristics of the eye.

Such determinations are subject to experimental errors and such eventsas accommodation of the eye to the substituted lens in a manner whichgives the impression that a particular correction is desirable, when infact that correction is not optimum.

Further, these measurement techniques determine corrections whichimprove overall vision, but are limited in normal practice to prism,cylindrical and spherical corrections which are low order corrections tothe patient's actual, detailed vision errors which include higher orderterms or characteristics which these measurement techniques cannotdetermine.

For the most part these prior art measurement systems are subjective andrequire the active participation of the patient for their success. Insuch cases the ophthalmologist must rely on the patient to indicateaccurately which images are clearer than others as an indication of theappropriate degree of correction. This requirement for activeparticipation of the patient is a disadvantage in a number ofcircumstances such as in the diagnosis of small children who havedifficulty in understanding what is being asked of them and prevents itsuse for infants who are incapable of indicating the effect of such lenssubstitutions.

The requirement for the active participation of the patient in thedetermination of the characteristics of the eye can have unfortunateeffects. Some anomalous conditions result in permanent disabilitiesbecause they are not detected during infancy because of the inability ofinfants to communicate with ophthalmologists. For example, if one eye isin focus and the other is severely out of focus during the time thebrain is developing its ability to interpret visual signals, then apermanent disability develops in which the out-of-focus eye is neverable to contribute usefully to the brain's image recognition because ofa lack of proper stimulation during the period in which the brain'simage interpretation functions became established. A person sufferingfrom this condition can tell with the affected eye whether the lines inan image are sharp or fuzzy, but cannot assimilate the perceivedinformation into an image. Present subjective refraction measurementsystems are incapable of determining the development of this conditionin infants because they cannot accurately diagnose the visual acuity ofthe eye without the active participation of the patient.

A number of objective refractometers have been developed in the hope ofovercoming these problems. However, each of these has had problems ordeficiencies of its own. One common deficiency is accommodation by theeye being measured. Another common problem is determining andmaintaining accurate alignment of the measurement system during themeasurement cycle, since any misalignment can cause inaccurate results.

In recent years, substantial interest has developed in using lasersculpturing, i.e. ultraviolet (UV) light laser ablation, to shape theanterior surface of the cornea as a means of providing corrected visionin place of the use of glasses or contact lenses. U.S. Pat. No.4,665,913, which is incorporated herein by reference, discloses a UVlaser scanning ablation technique for shaping a cornea in which a laserbeam which produces a small spot is scanned across the cornea to removea desired thickness of corneal material on each scan. The area scannedis increased or decreased on subsequent passes to scan each portion ofthe corneal surface a number of times which is proportional to thethickness of material to be removed at that portion of the cornea.

An alternative to the direct shaping of the corneal surface, is toessentially permanently attach a lenticule to the cornea with thelenticule being shaped to provide the desired vision correction. We say“essentially permanent” because the intention is to leave the lenticulein place permanently, unless some problem should develop which requiresits removal. Such lenticules themselves may be reshaped or re-profiledby laser ablation at the time of installation or subsequent thereto tocompensate for changes in the overall characteristics of the eye. Suchtechniques are disclosed in more detail in U.S. Pat. No. 4,923,467,entitled, “Apparatus and Process for Application and AdjustableRe-profiling of Synthetic Lenticules for Vision Correction” by Keith P.Thompson, which is incorporated herein by reference.

In addition to the advantages provided by eliminating the need for eyeglasses or contact lenses, both of these techniques are conceptuallycapable of providing substantial additional advantages in that eachshould, under proper control and with sufficiently detailed correctioninstructions, be able to produce fully asymmetric reshaping of thecornea in a practical manner, rather than being limited to the sphere,cylinder and wedge approximation mentioned previously. If the spatiallyresolved refraction data indicated by the scanning lasers is notavailable, the most effective plan may be to measure the preoperativespherical aberration of the corneal front surface and maintain this sameaberration during the laser sculpting of the cornea (see Schwiegerling1998). If the spatially resolved refraction data were available, themethod of Klein (Klein 1998) could be used to plan the optimal scanningpattern to sculpt the cornea.

However, in order to provide such detailed correction, there is a needfor measurement techniques which measure the shape of the cornea and theexisting refraction characteristics of the eye with the same detail andprecision as can be provided by the correction modality in order thatthe errors may be fully corrected in this manner.

A measurement system providing such a correction measurement should befast and should measure the eye's detailed refraction characteristicsreferenced to the cornea as a function of position across the dilatedpupil. This position-dependent measurement may be categorized as aspatially resolved refraction measurement because the refraction at eachmeasurement region (point) is determined in that local measurementregion independent of the refraction at other, non-overlappingmeasurement regions.

In Penney (U.S. Pat. No. 5,258,791) and He (He 1998) a flying spotspatially resolved objective autorefractometer is described which solvesa number of these issues, but has inherent limitations of its own. Forexample, the design requires a sequence of measurements be made as aflying spot is scanned. The resulting system is complex and is subjectto errors due to patient movement. A similar flying spot system could beconstructed using the manual psychophysical system described in (Salmon1998) which employs Smirnov's principal, which would have the samedrawbacks as the Penney system.

Recently, Hartmann-Shack lenslett arrays (also known as micro-lensarrays) have been used to measure the entire wave front aberration ofthe eye (Salmon 1998, Liang 1997, Liang 1994). This method has muchpromise for solving the problems associated with the previoustechniques. The basic idea for this method of wave front measurement isillustrated in FIG. 15. In FIG. 15 we illustrate an input laser sourceviewed by the eye such that it forms a diffuse point source at thefovia. This diffuse spot then acts as a point source as it exits theeye. As this wave front passes through the lenslett array, it is focusedon the CCD. If the eye had perfect optics, the wave front exiting theeye would form a plane wave. In this case the fovial point source wouldappear on the CCD as a regular array of points of light and would matchperfectly with the reference spot locations obtained during acalibration operation. In FIG. 16 we illustrate the effects of anaberrated wave front. Here the wave front is no longer a plane wave. Asthe wave front passes through each lenslett, the focused point on theCCD is deviated from the reference position according to the slope ofthe wave front at the lenslett. This deviation, dy, along with the focallength of the lenslett allows us to compute the local partial derivativeof the wave front. The wave front is reconstructed by integrating allderivatives computed for each lenslett in the CCD image. Details of thisreconstruction process are provided in the following paragraphs.

Implementations of these lenslett array systems have been limited toresearch laboratories for several reasons. First, the laser spot on thefovia must be in rather sharp focus on the CCD array to allow reliablemeasurements. In these laboratory systems, this is usually accomplishedby imaging through the subject's spectacle correction. In a clinicalsetting where exam time is important, finding which spectacle correctionto use for each subject may be time prohibitive. A more serious drawbackis the problem of point “cross-over” shown in FIG. 17. This occurs whenthe wave front has so much aberration that the fovia point sourcesassociated with a given lens is mistakenly assigned to a neighboringlenslett. When this happens, the sign and/or magnitude of the partialderivatives of the wave front will contain a huge error. Another issueis the choice of reference axis of the wave front aberration. It wasdemonstrated by Cui (Chi 1998) that different wave front aberrationsresult as various reference axes are chosen. Since clinicians will wantto register the wave front aberration data with the corneal surface andthickness data, this issue must be resolved for the situation where oneinstrument is used for corneal measurements and another is used foraberration measurement.

In the present invention, we solve each of the current drawbacks totypical Hartmann-Shack lenslett array based aberration measurement.First, we provide a simple focus adjustment mechanism which can accountfor at least +/−10 diopters of focus error. We also provide bothhigh-resolution and low-resolution wave front analysis paths, so we caneasily and effectively solve the point “cross-over” problem and increasethe total number of data points being processed per exam. The complexissue of registering the corneal surface measurements with theaberration measurement is automatically solved by integrating bothsystems and providing simultaneous measurements in a single exam.

In addition to these individual benefits over the existing state of theart in measuring the corneal front surface, corneal back surface andthickness, and wave front aberration, we integrate these usuallyseparate ocular measurement functions into a single instrument. Thisprovides the additional benefits of:

(1) A more economical system compared to the combined cost of individualinstrument.

(2) Reduced exam time since several measurements are madesimultaneously.

(3) Data and exams are integrated into the same computer and database.

SUMMARY OF THE INVENTION

A method for the simultaneous measurement of the anterior and posteriorcorneal surfaces, corneal thickness, and optical aberrations of the eye.The method employs direct measurements and ray tracing to provide a widerange of measurements for use by the ophthalmic community.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparent tothose skilled in the art from the following description with referenceto the drawings, in which:

FIG. 1 is a representation of the layout of the combined advancedcorneal topography/wave front aberration measurement system.

FIG. 2 is a process flow diagram showing an exemplary sequence ofevents, in accordance with the principles of the present invention.

FIG. 3 is a process flow diagram showing an exemplary determination of apupil contour, in accordance with the principles of the presentinvention.

FIG. 4 is a representation of a ray tracing for a light segment from asource through the cornea and back to a camera.

FIG. 5 is a process flow diagram showing the horizontal and verticalcross sections projected onto the eye, in accordance with the principlesof the present invention.

FIG. 6 is a representation of the corneal front surface measurementcoverage for three cameras.

FIG. 7 is a process flow diagram showing an exemplary determination of acorneal front surface, in accordance with the principles of the presentinvention.

FIG. 8 is a representation of the use of ray tracing to estimate thecorneal back surface and corneal thickness.

FIG. 9 is a process flow diagram showing an exemplary ray tracing tocompute corneal back surface and thickness, in accordance with theprinciples of the present invention.

FIG. 10 is a process flow diagram showing an exemplary determination ofa wave front aberration, in accordance with the principles of thepresent invention.

FIG. 11A is a process flow diagram showing an exemplary process ofestimating fovial spot locations shown in FIG. 10.

FIG. 11B is a process flow diagram showing an exemplary process ofdetermining offset shown in FIG. 10.

FIG. 11C is a process flow diagram showing an exemplary process offitting a surface to a wave front determined as shown in FIG. 10.

FIG. 12 is a representation of a conventional corneal topography system.

FIG. 13 is a representation of a conventional corneal topography systemwith a small keratoscope target to try to measure more data in theperiphery of the cornea.

FIG. 14 is a representation of the cross pattern being projected ontothe cornea.

FIG. 15 is a representation of the fundamental operating principals ofthe Hartmann-Shack wave front aberration measurement.

FIG. 16 is a representation of the effect of an aberrated wave front onthe Hartmann-Shack lenslett array/CCD image.

FIG. 17 is a representation of multi-resolution micro-lens arrays tosolve the problem of point cross-over.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention improves upon prior art by (1) using the provenaccuracy of the keratometric target in a novel way to obtain fulllimbus-to-limbus coverage on the corneal front surface; (2) obtainingsurface measurements of the corneal back surface using both a newprojected light and a new ray tracing approach; (3) obtaining cornealthickness information from the corneal surfaces; (4) obtaining wavefront aberration information for the entire eye using a multipleresolution micro-lens array technique; and (5) obtaining all thesemeasurements simultaneously.

One embodiment of the invention consists of the corneal measurementcomponents, the wave front aberration components, the computer, and thesoftware as described below.

In FIG. 1 we show a block diagram of the major components of theadvanced corneal topography/wave front aberration measurement (ACT/WAM)system. The system consists of a keratometric target source (K1) withillumination source (not shown), projected cross source (Cross), a diodelaser (Laser), neutral density filter (NDF), multiple resolutionmicro-lens arrays (MLA1 and MLA2) with imaging cameras (C4 and C5), afront view camera (C3), front view camera lens (L5), left- andright-view cameras (C1 and C2) oriented at a skew angle with respect tothe eye (Eye), left- and right-view camera lenses (L1 and L2), afocusing mechanism (FM), IR floodlamps (IR1 and IR2), relay lens system(L3 and L4), beam splitters (BS1, BS2, and BS3), focusing relay lens (L6and L7), mirrors (M1 and M2), and a computer (Computer) with videodigitizer card(s).

The light path for the aberration wave front measurement is as follows.The laser light is attenuated by a neutral density filter (NDF) so thatthe power entering the eye (Eye) is at a safe level according to ANSIZ136.1-1993. The laser then passes through the beam splitters (BS2 andBS3), the focusing mechanism (lenses L6 and L7 and mirrors M1 and M2),beam splitter (BS1), the relay lens (lenses L3 and L4), and finallyenters the eye. In the eye, the laser is (approximately) focused on thefovia where it is diffusely scattered. The focusing mechanism is used tobring the laser into reasonably sharp focus. This diffuse laser spotthen takes the role of a point source for the reverse (measurement)path. The light from the diffuse laser spot travels out of the eye,through the relay lens (L3 and L4), is reflected by beam splitter (BS1),goes through the focusing mechanism (lenses L6 and L7 and mirrors M1 andM2), and then is reflected by beam splitters BS1 and BS2 to themicro-lens arrays (MLA1 and MLA2). The lenses are selected so that theentrance pupil of the eye is conjugate to the micro-lens arrays and thediffuse spot on the fovia is conjugate with the micro-lens cameras (C4and C5). As the micro-lens array focuses the diffuse spot onto thecameras, deflections from that generated by a perfect plane wave aredetermined and from these deflections, the aberration wave front iscomputed. For the total aberration, the required deflection from thefocusing mechanism must be taken into account as described below.

The light path for the corneal measurements is as follows. Thekeratometric target source (K1) is reflected off the cornea and imagedby the left-, front-, and right-view cameras (C1, C2, and C3). Thekeratometric target source is turned off and the flood lamps (IR1 andIR2) and/or projected cross source (Cross) are turned on to allow thesame three cameras to image the horizontal and vertical cornealsurfaces, corneal thickness, and the pupil contour. These digitizedimages are processed as explained below.

Keratometric Target

The keratometric target provides a pattern which is reflected off thecornea and imaged by the left-, front-, and right-view cameras (C1, C2,and C3). The pattern used in the keratometric target can be any patternwhich is usually employed on corneal topography (Gills 1995). Thepreferred physical shape of the keratometric target is a cone. Thisshape has the advantage of yielding an image in good focus whenreflected off the eye and can facilitate any desired target pattern justas claimed for the single-curvature placido plate in U.S. Pat. No.5,864,383. An example of a suitable pattern is the polar checkerboardpattern of U.S. Pat. No. 5,841,511. Using the three camera views of thereflected keratometric target, we process the reflected image in anentirely different manner than that disclosed for the single viewdiscussed in U.S. Pat. No. 5,841,511. The front view camera (C3) alongwith the keratometric target provides the function of the usualreflection-based corneal topography system (Gills 1995). The left- andright- view cameras (C1 and C2) provide the ability to measure thecorneal front surface to the horizontal limbal margin. In FIG. 6 thisfeature is illustrated. Here the left-view camera measures the leftcorneal region, the front-view camera measures the central cornealregion, and the right-view camera measures the right corneal region.Note that there is significant overlap between the regions. This overlapallows the generation of a mathematical representation of the frontsurface of the cornea as described in the algorithms below. Theillumination source for the keratometric target is placed behind thetranslucent, conical body of the keratometric target to provide auniform back light. It is composed of an array of IR LED's which can beturned on and off quickly during an exam.

Multiple Resolution Micro-Lens Arrays

As indicated above, one of the main problems with using micro-lensarrays to measure the wave front aberration is the possibility of spot“cross-over”. This happens when there is so much aberration that thefocused spot from one lenslett from the array crosses over into theregion where the neighboring lenslett would normally image its focusedspot. To solve this problem, a low resolution micro-lens array is usedin addition to a high resolution micro-lens array. The low resolutionarray has a larger aperture and/or a shorter focal length compared tothe high resolution array. In FIG. 17 we illustrate the cross overproblem and show how it is solved with the use of the low resolutionarray. In the upper part of the figure, a highly aberrated wave frontcauses the lower two lensletts to be imaged outside their expectedlocation regions. In this case the points would mistakenly be identifiedas belonging with the wrong lenslett and would in turn lead to anincorrect estimate of the partial derivatives of the wave front. In thelower portion of the figure we show the situation when we have a lowerresolution array. In this case the aberrated position of the pointsfalls within the expected location regions and the correctcorrespondence is made with the lensletts. There could be severalstrategies for using these multi-resolution arrays. Our approach is touse the low resolution array to estimate the aberrated wave front, thenuse this estimate along with optical ray tracing analysis to locate theexpected position of the high resolution points. Details of thisalgorithm are presented below.

Sequence of Events for Exam

FIG. 2 is a process flow diagram showing an exemplary sequence ofevents, in accordance with the principles of the present invention.

In particular, as shown in FIG. 2, the process of acquiring all of thisdata and performing the analysis prior to display and/or saving the datais performed in the following sequence of events:

1. Operator selects which exam data to acquire (cornea and/or wave frontaberration).

2. Operator focuses and initiates an acquisition.

3. The front, left and right views of the cross and pupil contour areacquired.

4. The front, left and right views of the keratometric pattern areacquired.

5. The low and high resolution views of the wave front are acquired.

Key software components of the ACT/WAM system are: (1) pupil contour;(2) Horizontal and vertical sections of the cross projected onto thecornea; (3) Corneal front surface; (4) Corneal back surface; (5) Cornealthickness; and (6) Wave front aberrations of the eye.

Pupil Contour

FIG. 3 shows a process flow diagram showing an exemplary determinationof a pupil contour, in accordance with the principles of the presentinvention.

In particular, as shown in FIG. 3, the processing of the pupil contouris performed on all three of the left-, front-, and right-view images.Since the illumination for this acquisition is near IR, all irispatterns appear to have about the same intensity so no specialprocessing is required for light iris patients such as light blue, orfor dark iris patients such as dark brown. The steps of the algorithmare:

1. Find point inside the contour.

2. For all angles, theta, from zero to 360 do the following:

3. Scan in a radial direction from the point found in step 1 in thedirection of theta for a dark to light transition using a smoothedderivative operation. Store this point as a possible pupil contourpoint.

4. If not done scanning, continue with step 3 for next theta value.

5. Apply heuristics tests to remove false detects and move the contouras required.

6. Interpolate missing contour points.

7. Convert the contour from polar to rectangular coordinates.

8. Smooth the rectangular coordinates representing the pupil contourusing a zero-phase low pass filter on the individual x and ycoordinates.

Horizontal and Vertical Cross Sections Projected Onto the Eye

The processing of the horizontal and vertical cross sections projectedonto the eye is performed on all three of the left-, front-, andright-view images. The front-view image is processed for the horizontalsection and both the left- and right-view images are processed for thevertical section. In FIG. 4 we show a ray tracing of a light segmentfrom a source through the cornea and back to a camera. The source of thesegment is at point A, a distal edge strikes the cornea at point B andthe segment is refracted, a refracted proximal edge intersects the backcorneal surface at D and part of the light is reflected to point E. At Ethe proximal edge is refracted to the camera point F. From calibrationdata we know the direction of the rays AB and EF. This together with anestimate of the index of refraction of the cornea and an estimate of thecorneal front surface allow the ray tracing indicated in FIG. 4, and thesolution of the corneal back surface point and thickness at theneighborhood of BCDE.

FIG. 5 shows a process flow diagram showing the horizontal and verticalcross sections projected onto the eye, in accordance with the principlesof the present invention.

In particular, the steps of the exemplary process as shown are:

1. Specify the index of refraction, (about. 1.3771) and input thecorneal front surface.

2. Process the image to find the distal contour represented by point Bin FIG. 4.

3. Process the image to find the proximal contour represented by point Ein FIG. 4.

4. Use ray tracing to find the thickness of the cornea and the points onthe corneal back surface.

Corneal Front Surface

FIG. 6 is a representation of the corneal front surface measurementcoverage for three cameras, and FIG. 7 is a process flow diagram showingan exemplary determination of a corneal front surface, in accordancewith the principles of the present invention.

In particular, the image processing for the corneal front surface isperformed on all three of the left-, front-, and right-view images. Foreach image we find a set of point correspondences for a given point onthe keratometric target and a point on the image. That is, we find thelocation of the edges in the digitized keratometric images. This edgelocation data is stored in a M rows by N columns polar array such that Mis the number of concentric rings and N is the number of samples(usually 360—one for each degree) in the azimuthal direction. Thecorresponding keratometric target points are computed during acalibration stage using a known surface and inverse ray tracing. Oncethe point correspondence has been found for all three images, a singlesurface is found which incorporates the information from all threeimages as described in the following:

1. Initialize surface to sphere of radius 7.8.

2. Initialize the system of equations.

3. For data corresponding to all three cameras do the following:

4. For all points for this data set do the following:

5. Use the point correspondence to compute the partial derivatives ofthe surface.

6. Constrain the system of equations according to the partialderivatives.

7. If more points in this data set, get the next data point and go tostep 5.

8. If more data sets, get the next data set and go to step 4.

9. Solve the system of equations for the current surface.

10. Compare the current surface with the previous surface. If noimprovement or iteration count exceeded, then exit. Otherwise, go tostep 2.

This algorithm can be solved for any surface type. Traditional choicesare Zernike polynomials, Taylor polynomials, and splines. Because of itslocal and controllable nature, the preferred embodiment of the inventionuses Bsplines.

Ray Tracing to Compute the Corneal Back Surface and the Thickness

Prior to performing the reconstruction of the corneal back surface andcorneal thickness measurements, the corneal front surface, the pupilcontour from all three of the left-, front-, and right-view images, andthe projected cross data (corneal front and back surfaces and thicknessdata for the horizontal and vertical meridians) are known. Thefundamental geometry for the use of ray tracing to compute the cornealback surface and corneal thickness is illustrated in FIG. 8. In thisfigure we see that the rays from the three cameras which correspond tothe same point on the pupil contour trace through the cornea at separatepoints. Knowing the corneal front surface and the index of refraction ofthe cornea allows rays to be traced through the cornea and intersect thesame point at the pupil. The iterative application of this strategy canbe employed to estimate the thickness of the cornea and thus estimatethe corneal back surface.

FIG. 9 is a process flow diagram showing an exemplary ray tracing tocompute corneal back surface and thickness, in accordance with theprinciples of the present invention.

In particular, as shown in FIG. 9, the exemplary algorithm is asfollows:

1. Load the corneal front surface from the keratometric target data.

2. Load the corneal front and back surface and thickness data along thehorizontal and vertical meridians from the projected cross data.

3. Initialize the corneal back surface using the horizontal and verticalmeridian data from step 2.

4. Initialize the system of equations for the corneal back surface.

5. For each point on the pupil contour do the following:

6. Trace the rays from the three cameras which correspond to the pupilcontour.

7. Adjust the thickness of the cornea to better match the rays at thepupil contour point.

8. Constrain the system of equations according to the adjusted thicknessfound in step 7.

9. If there are more contour points, get the next contour point and goto step 6.

10. Solve the system of equations for the corneal back surface.

11. Compare the current surface with the previous surface. If noimprovement or iteration count exceeded, then exit. Otherwise, go tostep 4.

Wave Front Aberration

The wave front aberration is computed from the focus mechanism positionand both the high- and low-resolution micro-lens array images. Themicro-lens array images are processed one at a time. The first to beprocessed is the low-resolution array image. This is because there arefewer elements to process and it is less sensitive to wave frontaberration than the high-resolution array image. The wave frontestimated from the low-resolution array image is then used to estimatethe location of the high-resolution array image spots. This facilitatesa simpler and more robust processing step for the high-resolution arrayimage than is normally possible. The only difference between theprocessing algorithm for the low-resolution and high-resolution arrayimages is in the search region for the fovial point images. For thelow-resolution array image, we search in the neighborhood of thelensletts based on the calibration data (reference point locations) andfor the high-resolution array image we use the low-resolution arrayimage results as previously indicated. Because of this similarity, wedescribe only the low-resolution array processing.

FIG. 10 is a process flow diagram showing an exemplary determination ofa wave front aberration, in accordance with the principles of thepresent invention. FIG. 11A is a process flow diagram showing anexemplary process of estimating fovial spot locations shown in FIG. 10,FIG. 11B is a process flow diagram showing an exemplary process ofdetermining offset shown in FIG. 10, FIG. 11C is a process flow diagramshowing an exemplary process of fitting a surface to a wave frontdetermined as shown in FIG. 10.

In particular, as shown in FIGS. 10 and 11A to 11C, the algorithm forprocessing the array image is as follows:

1. Process the image to estimate the location of the fovial spotscorresponding to each lenslett.

2. Find the brightest local average inside each search regioncorresponding to a lenslett.

3. Find the centroid of the immediate neighborhood around the spot foundin step 2.

4. Determine the offset of the current fovial spot from the referenceposition.

5. Using the calibration data and the centroid found above, compute thedistance (in millimeters) in the x and y direction of the centroid fromthe reference spot location.

6. Use the offsets from step 2 to fit a surface to the wave front.

7. Initialize a system of equations.

8. For each fovial spot found add two equations to the systems ofequations. One for a constraint from the partial derivative with respectto x and one for a constraint based on the partial derivative withrespect to y.

9. Solve the system of equations.

This algorithm can be solved for any surface type. Traditional choicesare Zernike polynomials, Taylor polynomials, and splines. Because of itscommon use in optical wave front analysis, the preferred embodiment ofthe invention uses Zernike polynomials.

While the invention has been described with reference to the exemplaryembodiments thereof, those skilled in the art will be able to makevarious modifications to the described embodiments of the inventionwithout departing from the true spirit and scope of the invention.

What is claimed is:
 1. A method for determining wave front aberration,comprising: estimating a location of a fovial spot; finding a brightestlocal average in said fovial spot; finding a centroid of an immediateneighborhood around said fovial spot; determining an offset of saidfovial spot from a reference position; computing a distance of saidcentroid from said reference position; and fitting a surface to a wavefront using said determined offset.
 2. The method for determining wavefront aberration according to claim 1, further comprising: initializinga system of equations; and adding at least two equations to said systemof equations corresponding to said fovial spot.
 3. The method fordetermining wave front aberration according to claim 2, wherein: one ofsaid at least two equations corresponds to a constraint from a partialderivative with respect to x; and the other of said at least twoequations corresponds to a constraint based on a partial derivative withrespect to y.
 4. The method for determining wave front aberrationaccording to claim 3, further comprising: solving said system ofequations.